The present invention relates to a system for the continuous mass polymerization of SAN (styrenic-alkenylnitrile) copolymer, and more especially to both a process and an apparatus for continuous mass polymerization of SAN.
The copolymerization of vinyl monomers, particularly styrene and acrylonitrile as applied here, is well known, as are the processes for their production. However, there are still many inherent problems which have either been unsolved, or in many cases, solved unsatisfactorily.
In the case of SAN polymerization, it is known that precise control is essential to the producton of a product having acceptable properties. For example, if the acrylonitrile content of copolymer in a single product varies by more than about 4%, the different copolymers become incompatible with one another, resulting in an unacceptable, hazy product. Because styrene and acrylonitrile monomers polymerize at different rates, careful control is needed not only during the polymerization stage, but also during the subsequent purification stages. Especially in these latter processing stages, it is essential to maintain uniformity of the product, because even small amounts of copolymer product having a high acrylonitrile content can cause a yellow discoloration of the entire product, due to cyclization of adjacent pendant acrylonitrile groups upon heating of the copolymer, e.g., even during subsequent thermoforming steps.
Efforts to maintain this requisite precise control of the SAN copolymerization process have typically resulted in other disadvantages and/or limitations in the process. For example, the range of products which can be produced by a given process is often limited, in terms of the relative percentage of styrene and acrylonitrile monomer content of the copolymer, the percentage conversion of the monomers and the rate of monomer conversion into polymer.
One major problem in controlling the mass polymerization of SAN has been the effective removal of the heat of reaction from the polymerization system. A desirable means of doing this is by vapor condensation and reflux. Efforts have been made to design mass polymerization systems for SAN using this principle, e.g., the system described in U.S. Pat. No. 3,813,369. However, this system employs a particular type of mixing apparatus which is expensive not only to build, but especially in the energy requirements for its operation. Furthermore, vapor condensation systems of this type are slowly responsive to temperature fluctuations within the reactor and they also experience problems of plugging and/or fouling of the condenser and of vapor lock. Such systems have moreover not been characterized by a high degree of flexibility in the range of SAN copolymer products which can be produced with them.
A further problem which has been encountered in the art of SAN polymerization involves contamination of the product with black specks, which are thought to result from corrosion of the reactor walls. The solution has been to use reactors made of stainless steel, which adds measurably to the cost of any installation. The use of stainless steel reactors has also been required as a measure to prevent the formation of so-called "popcorn" polymer, which is not only an off-quality product, but contributes to the problem of plugging in the system, particularly in a vapor condensation cooling system.
Of equal importance to the polymerization stage itself in a SAN polymerization process is the post-polymerization treatment of the product, e.g., the removal of residual monomer from the polymer, known as devolatilizing the polymer. Once the polymer/monomer mixture leaves the reactor there is great risk of producing high-acrylonitrile-containing polymer, due to the uneven rates of polymerization for the two different monomer species and the high temperatures which are utilized for devolatilization. In conventional processes, devolatilization is typically carried out with thin film devolatilizing equipment, such as the so-called "Film Truder", which evaporates the liquid monomer very rapidly to minimize further polymerization. This thin film equipment, however, is relatively expensive and requires an inordinate amount of maintenance, e.g., at least once daily, because of its many moving parts and the extensive seals characteristic thereof. This maintenance requires, therefore, that the polymerization line be shut down or that some measure be taken to hold material upstream during maintenance. The result is an uneven quality of product and added expense for equipment and operation.
It would be desirable, therefore, if a mass copolymerization process for SAN were available which could attain a high quality, uniform product by means of improved reactor mixing, more precise temperature control, and less expensive and mechanically-simpler machinery. Advantageously, this system should be highly energy-efficient.